A PCT patent application WO9833631, which is published on Aug. 6, 1998 discloses a chain saw with a chain tensioning device, in which, the chain guide of the chain saw is fixed between two clamping plates and then fixed on the housing with them, and it can move longitudinally corresponding to the housing. Through a hole of the eccentric wheel and a rectangular hole on the housing, a bolt is tightened on the clamping plates which are guided and can move longitudinally on the housing, and its bolt head fixes the eccentric wheel on the housing. In order to tension the chain, first loosen the bolt and rotate the eccentric wheel, so that the bolt and clamping plates will move toward the direction of the chain guide top; when the chain has reached the expected tension, stop rotating the eccentric wheel, and then tighten the bolt again, so that the eccentric wheel and the clamping plates are fixed on the set position on the housing by force. This chain tensioning process is relatively complicated, and the fixation of the chain guide tends to loosen, which may cause the saw chain to be loose and increases the danger of the chain to jump from the chain guide. Therefore, this problem can only be solved by frequently conducting follow-up tensioning to the saw chain.
In order to solve the chain saw tensioning problem mentioned above, a Chinese patent CN1174841C, which is published on Nov. 10, 2004 publishes a chain saw, which has a chain tensioning device. This chain saw has a housing as well as a sprocket wheel installed on the housing and a chain guide aiming at this sprocket wheel, this chain guide extends from the housing and guides the saw chain engaged with the sprocket wheel in a circular manner, this chain guide can be installed in a manner that it can be movably adjusted longitudinally against the housing and sprocket wheel by a tensioning device, and the tensioning device has a stop fixed on the chain guide and an eccentric wheel which can be driven by the rotation of a rotating element. This rotating element is composed of two housing-like internal and external hand wheels, the internal and external hand wheels are inserted to each other and rotate against each other around a common axis, during which, the internal hand wheel move along the axis corresponding to the housing and it can be fixed on the chain guide, the external hand wheel and the eccentric wheel are connected to each other in a carry-rotation way, and the internal and external hand wheels, the external hand wheel and the housing are connected by an offside clipping connector, and when the saw chain has been tensioned, this offside clipping connector can ensure relative static connection between the internal and external hand wheels. Through the structure mentioned above, after its chain has been tensioned, the chain guide of the chain saw won't get loosed easily, which can guarantee effective tension of the saw chain. However, during the tensioning process, it requires separate operations to the internal and external hand wheels, which are complicated and the human-machine function is weak.
A Chinese patent CN1138620C has disclosed a chain saw, the chain saw includes a sprocket wheel movably installed on the housing which is used in the saw chain guided on the chain guide, an eccentric wheel that makes the chain guided move longitudinally and a chain tensioning device with a rotating device that makes the eccentric wheel rotate. of which, the rotation and connection device between the rotating device and eccentric wheel is constituted by a detent overload clutch, corresponding to the required chain tension to adjust its transposition clamping force. During the tensioning process of the chain, this structure only has to operate on one rotating device, and the chain can be tensioned and the chain guide can be clamped at the same time. It has convenient operation and strong human-machine function, but the detent overload clutch tends to be worn and the structure is not stable enough. In addition, during backward rotation of the rotating device to exchange the chain, efforts need to be made to overcome the clamping resistance between the detent elements of the detent overload clutch, and the operation is not easy.